The intestinal bacteria are capable of metabolic transformations which may be critical in the formation or degradation of certain known carcinogens. Various bacteria of the flora are capable of forming nitrosamines from progenator substituted amines and nitrite. The flora is also capable of N-dehydroxylating N-hydroxyfluorenyl-acetamide and N-hydroxyacetamidobiphenyl and thus of converting these proximate carcinogens into a form less likely to exert a carcinogenic effect. The demonstration that reduction of the nitro group in p-nitrobenzoic acid can be attributed exclusively to the activity of the intestinal flora suggests that the flora may have a critical role also in metabolic transformation of carcinogens containing the nitro group. Compounds of interest include the known carcinogens 2-mitrofluorene and 4-nitrobiphenyl and drugs such as metonizdazole and piridazole which, although having a useful therapeutic role, have been implicated in mutagenesis and in some carcinogenesis. The role of the flora in the metabolism of these compounds will be studied by comparing their metabolism in germfree and conventional rats as well as in rats specifically associated exclusively with intestinal bacteria capable of certain metabolic reactions. By examining metabolites of these compounds in the urine and feces of these rats as well as in cultures of isolated bacteria, the role of the flora in the metabolism of these compounds will be clarified. Since the reaction intermediates in the carcinogenic or mutagenic process are unstable and not apt to be found in urine or feces, use will be made of the testor strains of histidine requiring Salmonella mutants developed by Ames. We have found that these mutants can be associated with germfree and gnotobiotic rats. The mutants attain high concentrations in the colon and there register the revertant response which can be detected in the bacteria of the feces when the rats are fed carcinogens. These rats enable metabolic studies of carcinogens to be complemented with a means for detecting reactive intermediates occurring in the colon which may be responsible for the carcinogenic process.